Among signal transmitting/receiving devices, a fish finder typically transmits an ultrasonic pulse from a transducer into water, and then receives an echo signal that is the transmitted pulse reflected from a school of fish or the ocean floor with the transducer. The fish finder then analyzes the received signal to display an image of the school of fish or the ocean floor on an appropriate location of a display module.
For this reason, the followings are typically required for such a fish finder: a wide detectable range, and a high signal-to-noise (SN) ratio and a high time resolution of the echo signal. However, if a transmitting pulse width is decreased to cause the time resolution to be higher, the detectable range and the S/N ratio will also be reduced. On the other hand, if the transmitting pulse width is increased to increase the detectable range and the S/N ratio, the time resolution will be reduced.
Thus, Japanese Unexamined Patent Application Publication No. 2005-249398 (e.g., [0010]-[0016] and [0026]-[0031]) proposes that a desired detectable range, S/N ratio, and time resolution can be realized by using a chirp signal (i.e., a FM signal having a signal frequency that is swept with time) as the transmitting pulse, and applying a pulse compression to the echo signal with a matched filter.
Japanese Patent No. 3575252 (e.g., [0016]-[0056]) utilizes a similar technique in ultrasonic flaw detection for a metal plate or pipe. Further, for ultrasonic diagnostic devices, Japanese Unexamined Patent Application Publication No. 2003-325506 (e.g., [0001]-[0013]) proposes that a compensation for canceling frequency characteristics of a transducer is applied to a transmitting pulse in advance so that an ideal echo signal for the pulse compressing process is received with the transducer.
It is desirable that a drive circuit of a transducer for transmitting the pulse has a high power efficiency, and can be manufactured at low cost. Thus, Japanese Unexamined Patent Application Publication No. 2004-177276 (e.g., [0046]-[0056]) proposes that a transducer is driven by a PDM (Pulse Duration Modulation) signal.
FIG. 7A illustrates a drive circuit 50 of the transducer disclosed in Japanese Unexamined Patent Application Publication No. 2004-177276. FIG. 7B illustrates signals from respective components of the drive circuit shown in FIG. 7A, and the transmitting pulse. When gate signals shown in upper part of FIG. 7B are inputted into the respective gates (not illustrated) of four FETs (FET1-FET4), a PDM signal is applied to both ends of a transducer 1 to cause the transducer 1 to transmit a sinusoidal pulse as show in the bottom of FIG. 7B. A frequency of the transmitting pulse can be decreased/increased by increasing/decreasing cycles TA of the gate signals, and amplitude of the transmitting pulse can be increased/decreased by increasing/decreasing duty ratios TB/TA of the gate signals (here, TB is a pulse width of the gate signal).
FIG. 8 illustrates a transmitting pulse with a chirp signal that is generated as described above. Dashed lines are envelop curves of the transmitting pulse. A dynamic range of the transmitting pulse is represented by 20·log (VH/VL) (here, VH is the maximum amplitude value of the transmitting pulse, and VL is the minimum amplitude value).
However, as shown in FIG. 9, in the above-described conventional transmitting/receiving devices, because range side lobes periodically appear in the front and the rear of a main lobe of the pulse-compressed echo signal in the temporal axis direction, false images caused by the range side lobes are displayed on the display module. For example, the false images from the ocean floor caused by the range side lobes of the echo signal overlap on an image of a school of fish which is located in proximity to the ocean floor. As a result, the school of fish may not be detected.
As a technique for suppressing the range side lobes, it is known that a chirp signal (refer to FIG. 8) to which the amplitude modulation by a Gaussian function is applied is used as the transmitting pulse. In this technique, the more the dynamic range of the transmitting pulse is extended, the more the amplitude of the range side lobes can be decreased. However, because a realizable breadth of the dynamic range may be limited by performance of a transmitting module having the drive circuit 50 of the transducer 1, the suppression of the range side lobes by this technique may also be limited. For example, if the transducer 1 is driven by a PDM signal to transmit a pulse of 20 kHz carrier frequency, which is widely used for fish finders, the dynamic range will be limited to approximately 25-30 dB at maximum depending on a switching rate of the FETs. Further, waveform distortions (i.e., a difference between an ideal chirp signal waveform and a transmitting pulse waveform) may be caused to the transmitting pulse due to a finite amplitude/time resolution of hardware of the transmitting module, or frequency characteristics of the transducer 1. This also causes the increased range side lobes.
As another technique for suppressing the range side lobes, it is known that a product of “B” and “T” (hereinafter, simply referred to as a “BT product”) of the transmitting pulse having a chirp signal (“B” is a frequency sweep width of the transmitting signal, and “T” is a transmitting pulse width) is extended. In the fish finders, the transmitting pulse width is typically reduced to increase the transmitting cycle in a shallow water area. At this point, for suppressing the range side lobes, it is necessary that the frequency sweep width is increased to maintain the BT product to be greater than a predetermined value. However, increasing the frequency sweep width is limited by the frequency characteristics of the transducer 1. Further, this is not desirable because interferences between a transmitting pulse from a fish finder of one vessel, and a transmitting pulse from another fish finder of another vessel may be caused. On the other hand, if the transmitting pulse width is increased to maintain the BT product to be greater than the predetermined value, the following problem may be caused: electrical load of the drive circuit 50 of the transducer 1 is increased, or an echo signal detection range is narrowed in a shallow water area. The above-described two problems on the technique for suppressing the range side lobes may also be caused in other underwater detection devices (other than the fish finders), as well as in ultrasonic testing devices, ultrasonic diagnostic devices, or wireless radars, etc.